Jet diffuser for shrouded propellers



Sept. 5, 1961 H. R. CHAPLIN, JR

JET DIFFUSER FOR SHROUDED PROPELLERS 2 Sheets-Sheet 1 Filed April 29, 1959 FIG. I.

INVENTOR HARVEY R. CHAPL|N,JR.

p. 4.. RMM

ATTORNEYS p 1961 H. R. CHAPLIN, JR 2,998,700

JET DIFFUSER FOR SHROUDED PROPELLERS Filed April 29, 1959 2 Sheets-Sheet 2 42 M M 44 4e FIG. 2.

INVENTOR HARVEY R. CHAPLIN,JR.

BY 4? m fl. ATTORNEYS 2 998,700 JET DIFFUSER FOR SHROUDED PROPELLERS Harvey R. Chaplin, Jr., 3507 N. Ottawa St.,

. Arlington, Va. Filed Apr. 29, 1959, Ser, No. 809,891 p 2 Claims. (Cl. 60- 3'515) (Granted under Title 35, US. Code (1952 see. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The invention relates to a jet diffllSBI for shrouded propellers and more particularly to an apparatus for varying the amount of diffusion of the exhaust of a shrouded propeller.

In order to provide sufiicient thrust for take-off, climbing, and low speed operations, propellers are made larger than necessary for efi'icient high speed operations and/ or have controllable pitch devices. lf a shroud is placed around the outer edges of the propeller, the thrust of the system will be increased which may allow a reduction in propeller size and a more ideal propeller shape.

The increase in thrust due to a shroud depends upon the size attained by the slipstream as it flows away from the shroud. The larger the slipstream, the more thrust can be produced with a given power input.

The size or area of the slipstream, referred to here and in the ensuing discussion, is a hypothetical concept familiar to those skilled in the art. It is, namely, the

cross-section area which would be attained by the slipstream if the slipstream remained intact and did not mix with the surrounding air. The size of the slipstream is not directly, physically, measurable because of the mixing which does occur in actual cases.

Prior to this invention, larger slipstream size was attained by either making the propeller and shroud larger or by placing a diifuser (a diverging section of a shroud) behind the propeller. The angle of divergence (or diffusion) had to be small in order to avoid stalling or cavitation of the fluid flow within the shroud. Therefore, it the shroud was to promote a large increase in slipstream size, it had to be very long, have a large exhaust diameter, and have prohibitively high drag at cruising or high speeds.

I It is the primary object of this invention to disclose a jet difiuser for a shrouded propeller whereby the size of the slipstream may be controlled without stalling or cavitation, and without increasing the size of the shroud. Another object of this invention is to disclose a shrouded propeller system which has a large thrust during static or low speed operation and a low drag and high efliciency for high speed operation. Another object of this invention is to disclose a means for easily controlling the internal advance ratio of a shrouded propeller whereby a fixed pitch, constant 'r.p.rn. propeller may be used which operates at the optimum internal advance, ratio throughout the entire forward speed range of the vehicle.

I Other objects and advantages of the invention will hereinafter become more fully apparent from the following description of the annexed drawings, which illustrate a preferred embodiment, and wherein:

FIG. ,1 is a view, partly in elevation and partly in cross-section, of a preferred embodiment of the invention;

2 is a view, partly in cross-section and partly in elevation, of another embodiment of the invention "showing the jet diffuser mounter near the trailing edge of the shroud.

Referring now to the drawings, wherein like reference States atent O Patented Sept. 5, 196i;

characters designate like or corresponding parts throughout the several views, there is shown in FIG. 1 (which supplies shaft hors epower to the tiompressor 26 which in turn supplies compressed airby means of hollow supporting struts 28 to chamber 22. A H

The shrouded propeller system 10 may be attachedto the fuselage or wings of an airplane by extending the supporting struts 28 through the shroud 20 or by attaching the shroud or nacelle directly to an airplane.

During static operation (take-off) or low forward speeds of the propeller system :10, a high speed jet of air, indicated by number 30, is exhausted from the nozzle 24 and flows tangentially along the surface of the trailing edge of theshroud 29. This air jet 30 leaves the trailing edge in a direction approximately normal to the propeller axis (angle 6) and diffuses the exhaust of the propeller and shroud.

The static thrust of a shrouded propeller with a given power input and propeller efiiciency is approximately proportional to thegcube root of the ultimate cross-Section area, A at cross-section B, of the slipstream far downstream.

When the jet vdilfusenas in FIG. 1, is turned on at zero or low speeds, the ratio. of the slipstream areaddwnstream (section B) to the area of the exhaust at theexit of the shroud, A /Aex, can be increased far above where T=V4 A,P +J p A =the slipstream area downstream of the exit ofthe shroud. Aex=the slipstream; area at =the rate of'change ofmomentum or efifective thrust of the annular jet downstream of the exit of the shrongl". T=the total thrust of the propeller-shroud-jet' combination. 3 6=angle of divergence of the jet as it leaves the trailing edge of the shroud(see FIG. 1). g a p=density of the air or other workin g fluid. P=power supplied to the propeller multiplied by peller efiiciency. e a

, Themornenttim theory indicatesthat' a inajor port in of the jet momentum J is recovered asiis'eful thrust infthe form of higher suction pressures around the leadingedge of the shroud. f

This increase in thrust due to increasedjsli snesn size is greatest at the, static rendition and persisting tojs'tiine degree at all speeds, but becoming less important as the" forward, speed of the vehicle increases. j

A conventional diffuser, because of itsdr'ag, therefore becomes a liability at cruising or high speeds. The jet" diifuser, however, senses no appreciable increase in drag and therefore remains anasset at all forward spee s;

As an example, consider a IO-foot diameter "shrouded propeller of 91% efficiency, mounted on a 2-foot diameter nacelle (which extends for a short distance downstream of the shroud), driven by an engine with a power the exit a the t ead.

3 V of 1.1 million foot-pounds per second, operating statically in air of 0.00238 slug per cubic foot density.

Case l.-No diffuser.

Approximately:

T= V4 0.00238X 75.3X ==8950 1b. 10

In this example, by supplying 1,037 pounds of jet momentum to the jet difiuser, a total thrust increase 2,457 pounds was obtained.

An examination of the equation will also show that increasing the jet thrust as compared with the total thrust will further improve the area ratio and thrust increase.

The actual pressure in the above example, case 2, at area C just behind the propeller, will be about 63.4 pounds per square foot lower than at point B which indicates that the jet diffuser causes part of the velocity head at C to become converted to a pressure head at B to produce useful thrust.

An important advantage not exposed above is the following: the jet difiuser provides an effective means of controlling the internal advance ratio of the propeller, since at a given propeller thrust and forward velocity, the velocity of air passing through the propeller disc can be increased or decreased by increasing or decreasing the momentum J of the jet. It is thus possible to approach Case 2.Jet dt'fiuser with 6=90 and the highly attractive case of a fixed-pitch, constantrpm. propeller which operates at a favorable internal advance ratio throughout the entire forward-speed range of the vehicle.

Although the preceding discussion has confined itself to propellers using air as a working fluid, this invention is equally applicable to shrouded propulsive screws of surface or underwater marine vessels, the jet in this case being a high-speed jet of water derived from high-pressure water supplied to the annular chamber.

The location of the nozzle shown in FIG. 1, being 10- cated at the rearward extreme of the cylindrical inner portion of the shroud 20, is the presently preferred location because the jet then prevents flow separation from occurring inside the shroud near the trailing edge. However, the nozzle may equally well be located in the outer surface of the shroud near the trailing edge provided flow separation is prevented from occurring ahead of the trailing edge inside the shroud by some other means, such as by the auxiliary use of boundary-layer control techniques. For very high jet momentums, moreover, the induction effect of the jet itself will prevent such separation even with the nozzle located in the outer surface of the shroud.

This construction is illustrated in FIG. 2 where a nacelle 36 houses an engine 37 driving propeller 38 and compressor 39 and has hollow struts 40 connected to shroud 42. The annular chamber 44 at the rear of shroud 42 has an annular nozzle 46 facing outwardly from the shroud 42 at angle 6. V

In order to prevent flow separation between the slipstream and the jet stream from nozzle 46 due to the low 70 velocity layer next to the shroud, a small boundary layer control nozzle comprising a plurality of adjacent holes 48, facing towards the inner side and the rear of the 4 shroud 42, exhausts enough air to speed up the low velocity layer and prevent flow separation.

The amount of slipstream expansion which can be accomplished efficiently, even with a jet difiuser, is limited by the extent of low-energy wakes within the slipstream. Referring to FIG. 2, such wakes may arise, for example, from the struts 40 and the nacelle 36. If extremely large slipstream expansions are desired, it will be beneficial to remove these wakes (e.g. by suction through openings at the rear of the struts and nacelles) or to re-energizes them (e.g. by blowing air rearward through openings at the rear or along the sides of the struts and nacelles). A low-energy wake would normally arise from the shroud itself, but this wake is of no concern because it will be re-energized or speeded-up by the jets from holes 48 (FIG. 2) or absorbed, through induction, by the jet 30 (FIG. 1).

The jet difluser as thus disclosed provides a large increase in thrust at static conditions corresponding to a long shroud while requiring only a short, low drag shroud for use at high speeds.

It should be understood, of course, that the foregoing disclosure relates to only preferred embodiments of the invention and that it is intended to cover all changes and modifications of the examples of the invention herein chosen for the purposes of the disclosure, which do not constitute departures from the spirit and scope of the invention.

What is claimed is:

1. A jet diffuser for a shrouded propeller comprising a high speed engine having a relatively constant r.p.m., a fixed pitch propeller absorbing most of the power of said engine for propelling a fluid into a fluid stream, a fluid compressor mounted between said propeller and engine for generating a high pressure fluid, a streamlined nacelle mounted around said engine and compressor and faired with said propeller for minimizing the wake of said fluid stream passing over said nacelle whereby effective diffusion may be achieved, a relatively short annular shroud mounted around said propeller for increasing the suction pressure of said propeller, said shroud having an annular chamber connected with an opening in the trailing edge of said shroud for forming a nozzle for exhausting said high pressure fluid from said compressor into said fluid stream for diffusing said stream to an area greater than said shroud whereby a large increase in effective thrust may be provided by the the consequent fluid velocity reduction and increase in suction pressure at the leading edge of said shroud, a plurality of hollow supporting struts connected between said nacelle and shroud, and connecting means between said compressor and nozzle including said hollow struts for applying said high pressure fluid to said nozzle whereby the thrust of said shrouded propeller may be varied to eflectively vary the advance ratio of said propeller.

2. A jet diffuser for a shrouded propeller according to claim 1 and further characterized by said trailing edge having a plurality of holes on the inner side of said shroud for exhausting additional high pressure fluid into said stream for preventing flow separation between said fluid stream and said high pressure fluid from said nozzle.

References Cited in the file of this patent UNITED STATES PATENTS 1,954,437 Washburne Apr. 10, 1934 2,425,904 Vernon Aug. 19, 1947 2,609,659 Price Sept. 9, 1952 2,922,277 Bertin Ian. 26, 1960 2,937,823 Fletcher May 24, 1960 FOREIGN PATENTS 1,139,801 France Feb. 18, 1957 

